1. Technical Field
Embodiments of this disclosure generally relate to an image forming apparatus capable of forming an image having a plurality of gradation levels.
2. Related Art
Typical image forming apparatuses capable of forming an image having a plurality of gradation levels (hereinafter referred to as a multi-gradation image) generate gradation characteristic data by using a pattern for correcting gradation (hereinafter referred to as a gradation correction pattern). The gradation correction pattern has known gradation levels to perform gradation correction on image data of the multi-gradation image to be outputted, in order to stabilize image density of the multi-gradation image formed on a recording medium.
In such image forming apparatuses, for example, a gradation correction pattern having patches corresponding to a plurality of input gradation levels is formed on an intermediate transfer belt serving as an image carrier. The density of each patch of the gradation correction pattern is detected by a density sensor. According to a detected density of the gradation correction pattern, gradation characteristic data is generated that shows a relation between image density and gradation levels in a gradation range of the multi-gradation image that can be formed. The gradation is corrected upon formation of the multi-gradation image by using the gradation characteristic data.
When the gradation correction pattern having the patches is used, the patches of the gradation correction pattern are selected as appropriate so that the gradation is corrected as appropriate even when the gradation characteristics change significantly due to changes in the environment.
To correct the gradation as appropriate, some typical image forming apparatuses use a continuous gradation pattern as the gradation correction pattern, in which input gradation levels change continuously from a minimum gradation level to a maximum gradation level. In such image forming apparatuses, a density sensor continuously detects density of each portion of the continuous gradation pattern formed on the intermediate transfer belt that rotates at a predetermined speed, in a predetermined sampling period. In addition, an input gradation level of each portion of the continuous gradation pattern is calculated according to the speed at which the intermediate transfer belt rotates, the sampling period, and the length of the continuous gradation pattern formed on the intermediate transfer belt. Gradation characteristic data is generated according to the detected density of each portion of the continuous gradation pattern and calculated input gradation levels.
However, when the continuous gradation pattern is used as the gradation correction pattern, the accuracy of the gradation characteristic data may decrease due to variation in detected input gradation levels at the respective positions of the continuous gradation pattern at which the density is detected. The variation in the detected input gradation levels may be caused by, e.g., variation in the speed at which the intermediate transfer belt serving as an image carrier rotates and/or variation in the length of the continuous gradation pattern formed on the intermediate transfer belt.